The present invention is concerned with electromagnetic shielding in electronic devices, especially radio devices. The present invention has particular applicability to radiotelephone transceivers manufactured in the format of a PC card or PCMCIA card.
Electromagnetic signals are generated as a by-product during normal operation of an electrical circuit. Electromagnetic compatibility (EMC) is defined as the ability of a device to function properly in its intended electromagnetic environment and not be a source of unacceptable pollution to that environment. In some situations, shielding is required to provide electromagnetic compatibility between a sensitive or noisy part of the circuit and the rest of the circuit. One typical example is the isolation of a frequency synthesiser from the rest of the RF section in a cellular radiotelephone.
FIG. 1 shows a conventional approach to providing electromagnetic shielding in this situation.
Referring to FIG. 1, a radiotelephone housing 10 has located within it a printed circuit board 12. The printed circuit board 12 carries surface mount components. Some of these components 14, for example the frequency synthesiser, require shielding from other components 16, for example the rest of the RF section 16. To this end, a shield box 18 is used to enclose electromagnetically the components 14 and thus provide for EMC between the components 14 and the component 16.
However, because of the mechanical tolerances required in the height of the shield box 18, resulting from the manufacturing of the shield box 18 itself or the surface mount process, a gap must be designed for between the roof 18a of the shield box and the housing 10. This space plus the thickness of the roof 18a of the shield box 18 equates to the cost in terms of height which this conventional approach to shielding incurs. The `height cost` is illustrated in FIG. 1 by double headed arrow h1.
The present invention is concerned with reducing the height cost which this conventional approach inevitably incurs.